Ever since Foster City was created in the mid-20th century, a big chunk of land near the city’s center has sat vacant — first waiting years for a planned high school to arrive, then several development projects that never occurred.

Now the 15-acre chunk next to City Hall is poised to develop into a new, mixed-use town center with hundreds of housing units and ground-floor retail. For real this time.

Lennar Corp. this week bought the market-rate residential component of the development from master-planner The New Home Co., according to public records. The deal is a milestone for the project, and city officials expect Lennar — a huge, publicly traded homebuilder — to act quickly on building up to 200 for-sale housing units.

“The entitlements are all approved,” said Curtis Banks, community development director for the city of Foster City.

The project, called Foster Square, is located between Shell Boulevard and Foster City Boulevard at Civic Center Drive. It includes 150 senior assisted-living units; 200 age-restricted for-sale housing units (for people 55 years and up); 66 affordable units; and 30,000 square feet of retail.

Lennar is only buying the 200 age-restricted units. Mid-Peninsula Housing is biting off the affordable component. And Atria Senior Living is on board for the senior living project.

As for the retail? That will be built out on the ground floor of and developed by Blake Hunt Ventures, the Danville-based firm that has a long track record in mixed-use management and leasing.

“It’s intended to be a gathering spot for people,” Banks said. “It’s an area that’s meant for the entire community.”